The present invention relates to fan blades for gas turbine engines of the type referred to as turbofan engines. More particularly, the present invention relates to a composite blade having a skin transitioned to a metal semi-spar in such a manner that skin retention is not dependent on bond integrity, the composite blade of the present invention having a metal dovetail attachment incorporated into the blade.
A rotating blade, particularly one rotating at high speed, is subject to a number of forces which result in the blade experiencing bending stresses, tensile stresses, and torsional stresses. As an airplane moves through the air, the blades produce a thrust force caused by the airfoils producing lift as they rotate in a plane normal to the air inlet direction. This thrust force produces both bending stress and torque in the blades. A centrifugal force is created as a result of the rotation of the fan or propeller. This centrifugal force causes tensile stresses in the blade. Notwithstanding the strength requirements of such blades, it is further desirable to minimize the weight of such blades in order to reduce the overall weight of the engine.
It is known to produce blades for aircraft engines from composite materials such as, for example, graphite or boron or glass fibers or any combination thereof, in order to optimize the strength and weight requirements. However, it is believed that one of the critical areas of such blades is the point of attachment between the blade and hub assembly. In particular, in certain applications it is believed that the attachment requires a monolithic metal structure incorporated in the blade to support attachment. In a large turbofan engine, a dovetail attachment has proven to be an effective device for use as a load transition of a blade to a hub or disk and has been used on rotating blades of various styles and configurations since the invention of turbomachinery.
FIG. 1 illustrates a typical dovetail attachment, with the dovetail portion 2 of the fan blade 3 being secured to the disk 4 at dovetail to disk post interface 5. The dovetail portion 2 fits between disk posts 6 by sliding the blade 3 into disk 4.
Experience has shown that composite blades can be manufactured to meet virtually all requirements except those of adequate attachment transition. Since a composite blade is a collection of unidirectional fiber material, loading normal to the fiber is essentially limited to a matrix resin system. Since virtually all resins can withstand a limited amount of compression, very large surface areas are required for load distribution considerations if a dovetail configuration of composite material is desired. Such attachments have been conceived and tested over the past twenty years, but none have proven to have the durability to afford extended service. Experience has shown that a monolithic metal attachment is the only attachment proven to provide desired durability.
Thus, a composite blade having a metallic attachment has been the industry choice. This combination requires the composite to be bonded to the metal, i.e., a spar-shell design. FIGS. 2A and 2B illustrate a typical composite blade 6 having a shear area 8 which experiences shell to spar load. The blade spar 10 interfaces with the shell 14 made of composite plies at the shear surface 12.
Large areas of spar are required in order to transition the airfoil unidirectional load from the composite skin to the spar. A large area of spar results in much weight and machining cost being added to the process of manufacturing a composite blade. To prevent disbonding from age, impact, or manufacturing tolerances, one approach has been to secure the blade skin to spar by a secondary retentive feature. This securing is accomplished by a series of bolts and/or rivets to assure the skin load is transitioned to the spar. In FIG. 3, blade root 16 has its shell 14 secured to the spar 10 by a plurality of bolts 18 and rivets 20.
In light of the above discussion, it is evident that a need exists for a blade which incorporates a metal dovetail attachment and reduces spar cost, reduces spar area (bond area), and eliminates the need for an added secondary retention feature.